In POTS or other communication system it is advisable to perform a regular testing of communication lines, for example communication lines running from a central office terminal to end users (“subscribers”). With such a regular line testing, deteriorations or faults of the communication lines can be detected quickly so that a possibly necessary repair can be performed. In such a line testing operation, in particular electrical properties of the communication line like resistances are measured.
In FIG. 9 a conventional set-up for line testing is shown. A linecard 38 has a plurality of POTS circuits 39, i.e. respective POTS chipsets for driving a plurality of pairs of communication lines, each pair comprising a tip line 41 and a ring line 42. A backplane connection 49 serves to connect the POTS circuits 39 with further circuitry for data processing.
Associated with each of the POTS circuits 39 is a test relay 40 which serves to selectively connect one pair of communication lines 41, 42 with a test bus 43. This test bus 43 leads typically to a test board 44 comprising analog circuitry 45, a digital signal processor 46. a random access memory (RAM) 47 and a read-only memory (ROM) 48. The components on the test board 44 serve to perform various tests like resistance measurements on the communication lines 41, 42 selectively connected with the test bus 43 via the respective test relay 40.
This solution needs a test relay 40 for each POTS circuit 39, i.e. for each subscriber line, and a separate test board 44. For each test board 44 which is normally present only once in a given system only one subscriber line can be tested simultaneously.
An alternative approach is shown in FIG. 10. Here, a linecard 51 comprises a plurality of combined POTS circuits/digital signal processors (DSP) 50, each driving a subscriber line comprising a tip line 41 and a ring line 42. Again, the POTS circuits are connected via a backplane connection 49 with further circuitry. In this case, the test functions are integrated in the POTS circuits, for example in a respective subscriber line interface circuit (SLIC) or in a respective coder/decoder (CODEC). In known linecards of this type, only limited measurements can be performed on the subscriber lines since, for instance, an independent grounding of either the tip line 41 or the ring line 42 is not possible without additional circuitry, and therefore additional chip area is needed, causing additional costs. The method used here is based on applying a given voltage to the subscriber line and measuring the resulting current. With this method, only an overall resistance of the subscriber line can be measured.